fix(common): anchor a file group's first slice on a committed log (#19785)

A MOR file group can end up with its latest file slice keyed on a base
instant that never committed. For example, under NBCC with the bucket
index the earliest delta commit on a file group fails (or is rolled back)
while later delta commits on the same group succeed. Log files are
attributed to a slice by completion time, so when the group is first
built the failed instant's log arrives first and opens a slice keyed on
that uncommitted instant; every later committed log is then attributed to
the same slice.

isFileSliceCommitted only checks whether the slice's base instant itself
committed, so the whole slice -- including its committed log files -- is
treated as uncommitted and dropped from the reader view, silently losing
committed data.

Fix this at file-group construction instead of weakening the visibility
gate. When the group has no slice yet, the earliest completed log now
establishes the initial slice, so the slice is anchored on a real
committed instant. An earlier pending log then follows the normal
pending-log rule and attaches to that committed slice, instead of
creating its own uncommitted slice that would also hide every later
committed log in the group. isFileSliceCommitted keeps its original
single invariant, so all existing and future view APIs benefit without
per-call trimming.

addLogFiles now owns the sort and the re-anchoring for a batch of log
files; addLogFile becomes private and the file-system view calls the
batch API. Sorting is done once and reused by both the re-anchor scan and
the attach loop.

Tests:
- TestHoodieFileGroup#testUncommittedFirstLogDoesNotAnchorCommittedLogs
  verifies an uncommitted earliest log does not anchor the slice; the
  first committed log becomes the base instant and the pending log
  attaches to it.
- TestHoodieFileGroup#testUncommittedBaseFileSliceStaysHiddenDespiteCommittedLogs
  verifies a slice with an uncommitted base file stays hidden even when
  later committed logs land in the same file group.
- TestHoodieTableFileSystemView#testUncommittedFirstLogUsesFirstCommittedLogAsBaseInstant
  verifies the file-system view anchors the raw slice on the first
  committed log and surfaces only the committed logs.

Closes #19774